Conventionally, semiconductor lasers that can output light in various wavelength ranges, covering those from the visible light range to the mid-infrared range of wavelengths, have been studied and developed. However, a light source easily employed at room temperature has not yet been developed, for example, for the visible light range of wavelengths from 500 to 600 nm, or for the near-infrared to the mid-infrared range of wavelengths from 2 to 5 μm. Therefore, for a wavelength range where the direct emission of light by a light source is difficult, a light source for performing wavelength conversion employing nonlinear optical effects is used.
Various types of wavelength converters are known, and from a practical viewpoint, a waveguide type wavelength converter is most appropriate that employs periodic modulation of nonlinear optical constant for quasi-phase matching. As a method for constituting a periodical modulation structure, either the sign of a nonlinear optical constant is alternately inverted, or a portion where a nonlinear optical constant is large and a portion where it is small are almost alternately arranged. In a ferroelectric crystal such as LiNb03, since the positive or negative sign of a nonlinear optical constant (a constant d) corresponds to the polarity obtained by spontaneous polarization, the sign of the nonlinear optical constant can be inverted by reversing the spontaneous polarization.
FIG. 1 illustrates an arrangement of a light source that employs a conventional wavelength converter. The wavelength converter includes an optical waveguide 12 that is formed in a LiNbO3 substrate 11 and has periodically poled structure. A signal light of a wavelength λ1 and a pump light of a wavelength λ3 are emitted by two semiconductor lasers. The signal light and the pump light are combined by a multiplexer 13 and are entered to the optical waveguide 12. Since difference frequency generation is performed due to nonlinear optical effects, a converted light of a wavelength λ2 is output from the optical waveguide 12. The difference frequency generation is employed for the light source in this example, however, a light source may employ sum frequency generation or second harmonic generation and a light source which emits converted light of a short wavelength may be provided.
Assuming λ1, λ2 and λ3 are defined as wavelengths of a signal light (a first incident light), a converted light (an idler light) and a pump light (a second incident light) for a case of frequency difference generation, the following relationship is established for these three wavelengths.1/λ3=1/λ2+1/λ1  (1)When the two incident wavelengths are, for example, 1.55 μm and 1.06 μm, a converted wavelength of 3.55 μm can be generated. When the two incident wavelengths are 1.55 μm and 0.94 μm, a converted wavelength of 2.39 μm can be generated. Further, for a case of sum frequency generation, expression (1) is employed when λ1, λ2 and λ3 are defined as the wavelengths of the signal light (a first incident light), the pump light (a second incident light) and the converted light (a sum frequency light), and for a case of second harmonic generation, expression (1) is employed when λ1 (=λ2) is defined as the wavelength of the incident light and λ3 is defined as the wavelength of the converted light (a second harmonic light).
When n1 is a refractive index of a nonlinear optical material at the signal wavelength λ1, n2 is a refractive index at the converted wavelength λ2 and n3 is a refractive index at the excitation wavelength λ3, and Λ0 is defined as a modulation period for a nonlinear optical constant, a phase mismatch Δβ is represented by:Δβ=2π(n3/λ3−n2/λ2−n1/λ1)  (2)When L denotes the length of the waveguide of a wavelength converter, conversion efficiency η is represented by:
                    [                  Expression          ⁢                                          ⁢          1                ]                                                            η        =                              ηmax            ⁡                          [                                                sin                  ⁡                                      (                                                                  (                                                                              Δ                            ⁢                                                                                                                  ⁢                            β                                                    -                                                                                    2                              ⁢                                                                                                                          ⁢                              π                                                                                      Λ                              0                                                                                                      )                                            ⁢                                              L                        2                                                              )                                                                    (                                                            (                                                                        Δ                          ⁢                                                                                                          ⁢                          β                                                -                                                                              2                            ⁢                                                                                                                  ⁢                            π                                                                                Λ                            0                                                                                              )                                        ⁢                                          L                      2                                                        )                                            ]                                2                                    (        3        )            By referring to expression (3), the conversion efficiency η becomes highest when the phase mismatch Δβ is 2π/Λ0.
When the excitation wavelength λ3 is fixed, the signal wavelength that satisfies a quasi-phase matching condition of a phase mismatch Δβ=2π/Λ0 depends on wavelength dispersion of the refractive index of the nonlinear optical material, and substantially, is determined uniquely when the conversion period Λ0 is determined. When the excitation wavelength λ3 is changed from a wavelength (quasi-phase match wavelength) that satisfies the quasi-phase matching condition, the conversion efficiency is deteriorated in consonance with the expressions (2) and (3).
FIG. 2 illustrates a change in the conversion efficiency relative to a phase mismatch. In FIG. 2, the conversion efficiency η is normalized by employing a value of 1 as the maximum value. In a case of that an LiNbO3 substrate 50 mm long is employed as a wavelength converter, the wavelength band for a phase mismatch where the conversion efficiency η is half the maximum value is narrow, i.e., about 9.3 nm, when a wavelength is converted for a 3.35 μm band. As is apparent from expression (1), a plurality of different excitation wavelengths is required for the conversion of the signal wavelength λ1 to an arbitrary wavelength λ2. However, using a periodical modulation structure of a nonlinear optical constant at a constant period, a permissible range for a signal wavelength is narrow, and the signal wavelength can not be greatly changed. For example, in a case of that this wavelength converter is applied for a gas measurement apparatus that measures absorption of various types of gases and performs gas detection, it is preferable that wavelength sweeping be enabled for several wavelengths in order to measure a plurality of absorbed gases. However, a light source employing the conventional wavelength converter is not applicable for such operation.
On the other hand, well known is the fact that in a case of that a group velocity matching condition is established between converted light and signal light, or between converted light and pump light, a change in a propagation constant caused by a wavelength change is offset. Then, a change in a phase mismatch represented by expression (2) is moderated, so that phase matching can be performed for a broad wavelength band (see, for example, non-patent document 1). However, since this method depends on the dispersion of a nonlinear optical material that is in use, the method is employed only for a set of special wavelengths. There is another well known method for chirping the modulation period of a nonlinear optical constant in a quasi-phase matching wavelength converter (see, for example, non-patent document 2). By employing this method, a broadband wavelength converter can be provided for an arbitrary wavelength range, however the conversion efficiency is inversely reduced in proportion to the wavelength band. Therefore, in a case of that a high output is to be obtained in a wide range, the intensity of pump light or signal light must be increased, and an optical fiber amplifier, etc., is additionally required.
In a case of that continuous wavelength sweeping need not always be performed, phase matching is not required for a broad wavelength range, and a plurality of phase matching peaks corresponding to a plurality of excitation wavelengths must simply be obtained. Therefore, it is well known that a continuous phase modulation or a frequency modulation of a different period Λph is added to a structure having a modulation of a nonlinear optical constant with a period Λ0 (see, for example, patent document 1). It is also well known that phase modulation or frequency modulation is optimized to provide the maximum conversion efficiency for a periodical phase mismatch (see, for example, patent document 2). According to this method, a plurality of peaks are periodically included in wavelengths that are away from a wavelength having a phase mismatch Δβ=2π/Λ0 at a phase mismatch of 2π/Λph interval. Compared with a method for continuously broadening a phase matching curve, the conversion efficiency can be increased at the individual peaks.
However, in a case of that this method is applied for the above described gas measurement apparatus, the wavelengths that are required to be output for measurement are not always spaced equally. Even when optimization is performed to provide a peak for each of the wavelengths to be measured, an unwanted peak may be generated for measurement, and as a result, the conversion efficiency may be reduced at a phase matching peak that is required for the generation of a wavelength. Therefore, a problem encountered here is that when phase modulation or frequency modulation is simply added to the structure, as in the conventional manner, efficient wavelength conversion can not performed for a plurality of wavelengths that are not spaced equally.
An objective of the present invention is to provide a wavelength converter which performs simultaneously wavelength conversion for a plurality of input light wavelengths that are unequally intervals, and which causes little conversion efficiency deterioration, and a wavelength conversion apparatus of wavelength tunable type.    Patent Document 1: Japanese Patent Laid-Open No. 2004-20870    Patent Document 2: Japanese Patent Laid-Open No. 2004-233534    Non-patent Document 1: T. Yanagawa, et. al., Applied Physics Letters, Vol. 86, p. 161106, 2005    Non-patent Document 2: T. Suhara, et. al., IEEE J. of Quantum Electronics, Vol. 26, p. 1265, 1990    Non-patent Document 3: Y. Nishida, et. al., Electronics Letters, Vol. 39, p. 609, 2003    Non-patent Document 4: H. Ishii, Optical Fiber Communication Conference 2005 Technical Digest, Vol. 2, p. 91, 2005    Non-patent Document 5: M. Notomi, IEEE Photonics Technology Letters, Vol. 2, P. 85, 1990